U.S. patent application number 14/199548 was filed with the patent office on 2015-08-06 for high power ultra-short pulse laser-illuminated projector.
This patent application is currently assigned to IPG Photonics Corporation. The applicant listed for this patent is Valentin Gapontsev, Igor Samartsev, Alex Yusim. Invention is credited to Valentin Gapontsev, Igor Samartsev, Alex Yusim.
Application Number | 20150219986 14/199548 |
Document ID | / |
Family ID | 53754739 |
Filed Date | 2015-08-06 |
United States Patent
Application |
20150219986 |
Kind Code |
A1 |
Gapontsev; Valentin ; et
al. |
August 6, 2015 |
High Power Ultra-Short Pulse Laser-Illuminated Projector
Abstract
A laser illuminated projector system is configured with multiple
Red, Green and Blue laser sources. The Green laser source has an
all fiber master oscillator power amplifier configuration in which
pump light is coupled into the output end of the fiber amplifier in
a counter-propagation direction rendering the structure of the
Green source and therefore projector system compact. The Green
laser source is operative to emit signal light pulses at about 1064
nm wavelength with a pulse repetition reaching of up to about 3000
kHz, pulse duration between about a 100 fm to about 100 psec, an
average power between 1.5 W to above 30 W, a peak power above 5 MW,
a pulse energy exceeding 100 .mu.J and a beam quality parameter
M.sup.2 ranging between 1.2 and 1.5. The thus configured Green
laser source substantially reduces speckle otherwise visible on the
laser illuminated screen.
Inventors: |
Gapontsev; Valentin;
(Worcester, MA) ; Samartsev; Igor; (Oxford,
MA) ; Yusim; Alex; (Boston, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Gapontsev; Valentin
Samartsev; Igor
Yusim; Alex |
Worcester
Oxford
Boston |
MA
MA
MA |
US
US
US |
|
|
Assignee: |
IPG Photonics Corporation
Oxford
MA
|
Family ID: |
53754739 |
Appl. No.: |
14/199548 |
Filed: |
March 6, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61935241 |
Feb 3, 2014 |
|
|
|
Current U.S.
Class: |
353/31 |
Current CPC
Class: |
G03B 21/2013 20130101;
G03B 21/204 20130101; G03B 21/208 20130101; G03B 21/2033
20130101 |
International
Class: |
G03B 21/20 20060101
G03B021/20 |
Claims
1. A laser illuminated projector system, comprising: multiple laser
sources emitting Red, Green and Blue ("RGB") light beams, the Green
laser source having a master oscillator/power amplifier ("MOPA")
structure including: a fiber seed laser operative to emit
sub-nanosecond pulses of single mode ("SM") signal light at a
fundamental frequency in a propagating direction along a light
path, a fiber booster provided with a light-emitter-doped multimode
core ("MM") which has a bottleneck-shaped cross-section expanding
toward a downstream thereof and receiving pulses of the SM signal
light, and a reflective element located downstream from the fiber
booster and configured to receive and reflect pump light into the
downstream of the MM core in a counter-propagating direction,
wherein the MM core is configured to emit sub-nanosecond pulses of
amplified signal light at the fundamental frequency in
substantially a fundamental mode, and a single pass second harmonic
generator ("SHG") receiving the pulses of amplified signal light
and configured to double the fundamental frequency so as to
generate sub-nanosecond pulses of Green light; and an RGB projector
head configured to receive the RGB beams.
2. The laser-illuminated projector system of claim 1, wherein the
Green laser source is operative to emit signal light pulses at
about 1064 nm wavelength with a pulse repetition reaching of up to
about 3000 kHz, each pulse having duration between about a 100 f to
about 100 psec, an average power between 1.5 W to above 30 W, a
peak power above 5 MW, a pulse energy exceeding 100 .mu.J and a
beam quality parameter M.sup.2 ranging between 1.2 and 1.5.
3. The laser-illuminated projector system of claim 1, wherein the
fiber booster includes a uniformly dimensioned relatively long
upstream fiber section and a relatively short downstream fiber rod
section terminating upstream from the SHG, the MM core extending
through an entire length of the fiber booster and having an
upstream uniformly dimensioned portion which co-extends with the
fiber section and receives the SM signal light from the seed laser,
an adiabatically expanding mode transforming portion and uniformly
shaped amplifying portion which has a larger cross-section than
that of the upstream portion, the amplifying portion of the MM
fiber extending through the fiber rod part section of the fiber
booster.
4. The laser-illuminated projector system of claim 3, wherein the
fiber portion of the MM core is dimensioned so that the SM signal
light from the fiber seed laser excites only the fundamental mode
upon coupling into the fiber booster, the mode transforming portion
of the MM core being configured to adiabatically expand the
fundamental mode without excitation of high order modes.
5. The laser illuminated projector system of claim 3, wherein the
fiber booster has a monolithic splice-less structure or includes
the fiber and fiber rod sections spliced to one another.
6. The laser illuminated projector system of claim 3, wherein the
Green laser source further includes: a pump source configured to
output the pump light at a pump wavelength shorter than a
wavelength of the signal light from the seed laser and provided
with at least one or more MM passive pump light delivery fibers; a
flexible cable dimensioned to be traversed by the fiber booster and
pump light delivery lights and terminating upstream from the SHG; a
silica-made buffer located upstream from the reflective element and
having an upstream face which is fused to output ends of respective
fiber booster and pump delivery fiber.
7. The laser illuminated projector system of claim 6 further
comprising: a portable laser head housing the silica-made buffer,
reflective element and SFIG, and a main console housing the seed
laser and pump source, the flexible cable extending over free space
between the main console and laser head.
8. The laser illuminated projector system of claim 7 further
comprising a first chirped upstream volume Bragg grating located in
the laser head between the reflective element and SHG and
configured to compress pulses of the signal light at the
fundamental frequency.
9. The laser illuminated projector system of claim 7 further
comprising a second chirped volume Bragg grating configured to
stretch Green light pulses which are backreflected in the
counter-propagation direction downstream from the laser head at the
doubled frequency.
10. The laser illuminated projector system of claim 1, wherein the
seed laser is configured with a fiber laser cavity which is defined
between a semiconductor saturable absorber and a weak fiber Bragg
grating ("FBG").
11. The laser illuminated projector system of claim 10, wherein the
seed laser includes a linearly chirped FBG within the cavity
operative to stretch pulses of the signal light.
12. The laser illuminated projector system of claim 11, wherein the
seed laser further includes a piezo actuator mounted to a holder,
the holder being configured to support the chirped FBG and
operative to linearly and arcuately stretch the FBG in response to
a signal from the piezo actuator.
13. The laser illuminated projector system of claim 12, wherein the
holder is configured to stretch the chirped FBG to a desired length
so as to adjust a pulse duration within a pulse train.
14. The laser illuminated projector system of claim 4, wherein the
seed laser includes an output passive SM fiber spliced to the
upstream fiber section of the fiber booster, a core of the output
passive SM fiber and upstream portion of the MM core being
configured to support respective single and fundamental modes
having which have matching mode field diameters, respectively.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application relates to a US provisional application
entitled "HIGH POWER ULTRASHORT PULSED FIBER LASER" with both
applications being simultaneously filed.
BACKGROUND OF THE DISCLOSURE
[0002] 1. Field of the Disclosure
[0003] This disclosure relates to pulsed, broad-line fiber lasers.
More particularly, the disclosure relates to pulsed broad-line
fiber lasers for speckle reduction.
[0004] 2. Prior Art
[0005] The advantages of using laser sources for the illumination
of projection displays are compelling in comparison to using other
light sources. They include: improved image brightness, improved
power efficiency, improved image contrast, wider color gamut,
reduced cost, reduced size of both the illumination source and the
optical system, and improved depth of field for focus-free
usage.
[0006] The main disadvantage of using laser sources is a
potentially severe degradation of projected image quality due to
the presence of a high contrast, high spatial frequency, granular
pattern that seems to float at in front of the projected image
plane. This pattern is known as speckle.
[0007] Since speckle arises due to the highly coherent nature of
laser illumination, one means of improvement is to use sources with
reduced coherence, such as the direct-emission green laser diodes.
However, their linewidth is too narrow to reduce speckle to
acceptable levels. Nor is the power of the most powerful laser
diodes sufficient. The brightest and most power-efficient green
lasers available today do not have sufficiently broad
linewidth.
[0008] In recent years, the speckle suppression in laser projectors
is often obtained by the use of a multimode fiber. The length of
fiber should be long enough to achieve de-correlation of
practically all fiber modes. The greatest advantage of this method
is that it does not require mechanical movement. However, the
correlation length of incident laser beam on screen increases by a
factor equal to the magnification of the optical system. Therefore,
to preserve the same speckle contrast on the screen as that at the
distant fiber end, the method requires the number of multimode
fibers be approximately equal to the square of the magnification of
the projector objective lens (approximately equal to the number of
pixels on the screen >300000). Despite significant efforts to
develop a compact system for decreasing speckle noise to an
acceptable for the human eye level, this problem still
persists.
[0009] Hence there is a need for a fiber laser-illuminated
projector system with reduced speckle noise that is of a simpler
construction and more compact.
SUMMARY OF THE DISCLOSURE
[0010] In accordance with the disclosed subject matter, a fiber
laser-based system is provided to overcome the above described and
other deficiencies of the known prior art. In particular, the
disclosed system has demonstrated the reduced speckle effect.
[0011] This is achieved through a combination of efficient,
high-brightness, tunable red, green, and blue (RGB) pulsed laser
system designed for high-performance digital projection systems.
The result is a compact, all fiber light source that helps minimize
a speckle effect, has low power consumption, provides saving on
electricity as well as on heating, ventilation, and
air-conditioning (HVAC) operating costs.
[0012] The disclosed laser system is characterized by high peak
power and ultra-short pulses provided. Both these characteristics
in combination provide efficient conversion of the fundamental
frequency into the desired frequency of Green light. Individually,
ultra-short pulses cause the broadening of linewidth of signal
light at the fundamental frequency emitted from a seed source and,
therefore, speckle reduction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The features and advantages of the disclosed system will
become more readily apparent from the following description
accompanied by the drawings, in which:
[0014] FIG. 1 is a laser-based projection configuration of the
disclosed system.
[0015] FIG. 2 is an optical schematic of the disclosed high power,
ultra-short pulse fiber laser system;
[0016] FIG. 3 is an exemplary optical schematic of a seed
source;
[0017] FIG. 4 is an exemplary view of a pulse stretcher,
[0018] FIG. 5 is an exemplary optical schematic of the fiber laser
system of FIG. 2;
[0019] FIG. 6 is a diagrammatic view of the downstream end of the
booster of the laser system illustrated in FIG. 5; and
[0020] FIG. 7 is a diagrammatic view of the laser head of the laser
system shown in FIG. 5.
SPECIFIC DESCRIPTION
[0021] Reference will now be made in detail to the preferred
embodiments of the disclosure. Wherever possible, same or similar
reference numerals are used in the drawings and the description to
refer to the same or like parts or steps. The drawings are in very
simplified form and are not precise to scale.
[0022] The disclosed laser-illuminated projector 10 is configured
as a compact assembly due to a compact Green pulsed laser fiber
source with a single-pass frequency doubling conversion scheme. The
compactness of the Green laser source is a result of an all fiber
master oscillator power amplifier ("MOPA") configuration in which a
booster amplifier is end-pumped in a counter-propagating direction.
The system 10 is efficient and its high-brightness, tunable red,
green, and blue (RGB) laser technology is configured to output
ultra-short pulses with pulse duration in a femtosecond to
picosecond range and linewidth between about 0.5 to several
nanometers. The result is a compact light source that substantially
minimizes the appearance of speckles, has a long lifetime, and low
power consumption, saving on electricity as well as heating,
ventilation, and air-conditioning operating costs.
[0023] FIG. 1 illustrates a general exemplary schematic of
disclosed laser-illuminated projector system 10. The system 10 is
configured with three laser light sources 12, 16 and 18 emitting
respective green, red and blue signal beams and together
constituting a laser source assembly. The signal beams are guided
via respective bulk focusing optic over free space or output
passive fibers 20 and can be combined in the known to one of
ordinary skill manner in a Red Green Blue ("RGB") projector head 21
illuminating a screen 23.
[0024] Referring to FIG. 2, green laser light source 12 has an all
fiber configuration which considerably reduces the overall
dimensions the laser source assembly by comparison with other known
laser source assemblies used in cinema. Configured as a MOPA,
source 12 thus includes a seed 22 emitting short pulses of light in
a 1 .mu.m wavelength range. The pulses are further amplified in a
high power fiber booster stage 24 and finally compressed to the
desired duration by means of a volume Bragg grating 13. Utilizing a
single pass frequency doubling scheme 14 with a non-linear crystal,
such as lithium triborate ("LBO") crystal or any other suitable
crystal known to one of ordinary skill, the signal light at a
fundamental frequency is converted to Green light at the desired
frequency.
[0025] Turning now to FIG. 3, seed 22 may have a variety of optical
geometries well known to one of ordinary skill in the art and
depending on whether a passive mode locking technique or active
mode locking technique is used. For example, as shown in FIG. 3,
the passive mode-locking of seed 22 is achieved with a saturable
absorber mirror ("SESAM").
[0026] The SESAM can be used to mode-lock a wide range of laser
cavities. Pulses result from the phase-locking (via the loss
mechanism of the saturable absorber) of the multiple lasing modes
supported in continuous-wave laser operation. The absorber becomes
saturated at high intensities, thus allowing the majority of the
cavity energy to pass through the absorber to a Bragg-mirror 30,
where it is reflected back into the laser cavity limited on the
weak side by a fiber Bragg grating 38. At low intensities, the
absorber is not saturated, and absorbs all incident energy,
effectively removing it from the laser cavity resulting of
suppression of possible Q-switched mode-locking.
[0027] The Bragg-mirror 30 is mounted on a semiconductor wafer 32
like GaAs, covered by an absorber layer and a top film system,
determining the absorption. Although semiconductor saturable
absorber mirrors have been employed for mode-locking in a wide
variety of laser cavities, the SAM has to be designed for each
specific application. The differing loss, gain spectrum, internal
cavity power, etc., of each laser necessitates slightly different
absorber characteristics. An active fiber 36 in the seed laser
cavity is doped with ions of rare earth elements, such as
Ytterbium, and pumped by light emitted from a pump 38, which may
include diode lasers or fiber lasers which pump active fiber 36 in
the known manner, in the desired wavelength range around a 975 nm
center wavelength.
[0028] In order to extract more energy by the ultrashort pulses
from the amplifier a method called chirped pulse amplification is
used. One method to stretch the pulses in the disclosed monolithic
package is to use a linearly chirped fiber Bragg grating
("FBG")/broadband reflector 34. The FBG 34 can be designed to
stretch pulses up to 1000 psec or longer from 0.1 psec or shorter
into a femtosecond region. The linearly chirped FBG 34 is
particularly advantageous for fast switching.
[0029] Referring to FIG. 4 in combination with FIG. 3, the
adjustment of the pulse duration in real time is realized by a
tunable pulse stretcher 44 that uses mechanical flexure mount 42
that holds linearly chirped FBG 34. The piezo is located in
position to optimize the stretching of the FBG about a flexure
point. In contrast to the known piezo-based structures, having a
limited displacement which necessitates the use of a nonlinearly
chirped FBG for limited pulse duration adjustment, piezo mount 42
induces a much larger changes in the length of FBG 34 held in mount
42. These changes over the range of piezo operation permit the use
of a linearly chirped FBG to tune the pulse duration from 0.5 psec
to 5 psec and with obvious to one of ordinary skill in the laser
arts modifications this range can be extended even further from 500
fs to 30 psec with switching time as short as microseconds. The
switching frequency of pulse stretcher 34 generally depends on two
factors: operational frequency of a piezo element which reaches a
MHz level and material of mount 42. Thus selecting the material
that meets given requirements, pulse stretcher 44 is capable of
operating in a switching frequency range between KHz and hundreds
KHz. Accordingly, if a need arises to change pulse duration between
adjacent pulses, pulse stretcher 44 can satisfy this need. It
should be noted, that pulse stretcher 44 as disclosed above can be
used in any chirped pulsed amplification laser system.
[0030] Turning to FIG. 5 in combination with FIG. 2, the overall
layout of Green laser system includes a main console 46 housing
seed 22, one or more pump sources 13, optional pre-amplifying
cascade(s), electronics, cooling systems, power feed supply for the
seed laser and pump sources, and all other necessary components
cumulatively denoted as 35.
[0031] The SM signal light emitted by seed laser 22 is coupled into
a SM passive output fiber 48 delivering the SM signal light to an
active fiber 60 of the fiber booster with a double clad
configuration which surrounds a MM core. The active fiber 60
traverses a flexible delivery cable 50 extending over free space
between main console 46 and a laser head 52. The MM core of active
fiber 60 is doped with one or more light emitters, such as ions of
ytterbium ("Yb"). One or more passive multimode pump light output
fibers 58 extend over free space between console 46 and laser head
52 within cable 50. The output SM passive fiber 48 of seed source
22 is spliced to active fiber 60 within cable 50.
[0032] Turning briefly to FIG. 6, active fiber 60 may be configured
with separately manufactured uniformly dimensioned fiber 64 and
bottleneck-shaped fiber rod 66 fused together, but is preferably
manufactured as a monolithic, continuous one piece structure. The
signal light is amplified to the desired level reaching MW peak
power levels, if necessary, as it is emitted from booster stage 24
with an M.sup.2 beam quality parameter of the emitted signal light
varying between 1.1 and 1.5.
[0033] The MM core 75 of active fiber 60 is configured with at
least three regions: an input uniformly dimensioned region 68, a
frustoconical mode transforming region 70 and output amplifying
region 72. The excitation of only the fundamental mode in doped MM
core 75 is realized by initially matching a mode field diameter
("MFD") of the fundamental mode of MM core 75 with that of output
passive fiber 48 (FIG. 5) of the seed laser. Accordingly, signal
light coupled into active fiber 60 of booster stage 24 excites
substantially only a fundamental mode. The latter further
adiabatically expands as it is guided through transforming region
70 and amplified in amplifying region 72.
[0034] Referring to FIGS. 5 and 7, laser head 52 houses a
reflective element 62 such as a mirror. An opening 74 provided in
mirror 62 and centered on the optical axis of system 10, is
dimensioned to prevent or minimize signal light losses in the
propagating direction A. Preferably, opening 74 has a diameter
twice as large as a beam diameter, but may be somewhat larger, for
example, thrice the diameter of the beam waist. The diameter of
mirror 62 is substantially the same as a distance between the
downstream facet of active fiber 60 and opening 74. The mirror 62
is configured to redirect pump light emitted from pump fiber(s) 58
back into the output end of active fiber 60 in the
counter-propagating direction, as discussed in greater detail
below.
[0035] The laser head 52 further also houses buffer 76 having its
input fused to the output end of all fibers 60 and 58. The buffer
76 may be configured as a silica-glass coreless rod and operative
to prevent the damage to fiber ends due to the reduced power
density of the output beam propagating through the volume of buffer
76. The remotely positioned and easily displaceable laser head 52
eases the deployment of the Green laser system. However, laser head
52 may be installed within the main console.
[0036] The pump light delivery fiber 58 is configured as a passive
MM fiber. Preferably, a downstream end region of pump fiber 58
extends parallel to the output region 40 of active fiber 60. The
output ends of respective fibers 58 and 60 may be directly bonded
to the upstream face of buffer 76 along the propagation direction
A. Other spatial relationships between these two fibers are also
within the scope of the disclosure. For example one of the delivery
and active fibers can be bonded to the upstream face of the buffer
at an angle relative to the optical axis of the other. More than a
single delivery fiber can be used in combination with active fiber
60.
[0037] The disclosed Yb fiber laser including seed 22 and booster
24 operates at an average power varying between 1.5 W to above 30 W
and a peak power above 5 MW. The pulse energy may exceed 100 .mu.J
and be as low as 5 .mu.J. The beam quality ranges between 1.2 and
1.5. The pulse duration covers a 100 fs to 100 psec interval, and a
pulse repetition rate can reach 3000 kHz and higher.
[0038] Referring to FIGS. 1, 2 and 5, laser head 52 may be
configured with an extension 78 housing a compressor 13 (FIG. 2)
configured as a volume Bragg grating which is operative to reduce
the pulse duration to its original duration before the latter is
frequency doubled in second harmonic generator scheme 14 to output
Green light in a 515-545 nm wavelength range which by means of bulk
optics delivered to the RGB projector head.
[0039] The generated ultra-short Green light pulses are
characterized by an ultra-high peak power that may potentially
threaten the entire Green light module integrity because of
backreflection from screen 28 (FIG. 1). To prevent such a
possibility, system 10 incorporates anther volume Bragg grating 80
(FIG. 5) at the wavelength of Green light to stretch pulses
reflected from screen 20 lowering high peak powers of the
backreflected light.
[0040] The Blue and Red laser light sources may be configured as
diode lasers. Alternatively, nonlinear effects such as a
sum-frequency scheme incorporating the disclosed Yb laser can be
used to generate the desired color.
[0041] Although the present disclosure has been described in terms
of the disclosed example, numerous modifications and/or additions
to the above-disclosed embodiments would be readily apparent to one
skilled in the laser arts without departing however from the scope
and spirit of the following claims.
* * * * *